Recently, interests in energy storage technologies have increased. As the energy storage technologies are relevant to cellular phones, camcorders and notebook PC, and further to electric vehicles, the demand for a battery with high-energy concentration as a power source of such an electronic device has increased. A lithium ion secondary battery is one of the most satisfactory batteries, and many studies are now in active progress.
Among the secondary batteries currently in use is a lithium secondary battery developed in the early 1990's including an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.
Such lithium secondary battery has an average discharge voltage of about 3.6 to 3.7V, which is advantageously higher than those of other batteries such as alkali batteries or nickel-cadmium batteries. To give such a high operation voltage, an electrolyte composition that is electrochemically stable in a charging/discharging voltage range from 0 to 4.5V is required. For this purpose, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate or propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate are suitably mixed is used as a solvent of electrolyte. A solute of electrolyte commonly uses a lithium salt such as LiPF6, LiBF4 and LiClO4, which acts as a source for supplying lithium ions in a battery and thus enables the lithium battery to operate.
Lithium ions coming out from a cathode active material such as lithium metal oxide during an initial charging process of a lithium secondary battery are moved to an anode active material such as graphite and then intercalated between layers of the anode active material. At this time, due to the strong reactivity of lithium, electrolyte reacts with carbon of the anode active material on the surface of the anode active material such as graphite, thereby generating compounds such as Li2CO3, Li2O and LiOH. These compounds form a kind of SEI (Solid Electrolyte Interface) film on the surface of the anode active material such as graphite.
The SEI film plays a role of ion tunnel, which allows only lithium ions to pass. Due to the effects of ion tunnel, the SEI film prevents an organic solvent molecule moving together with lithium ions in the electrolyte solution and having a great molecular weight from being intercalated into layers of the anode active material and thus breaking down the anode structure. Thus, since the electrolyte solution is not contacted with the anode active material, the electrolyte solution is not decomposed, and also an amount of lithium ions in the electrolyte solution is reversibly kept, thereby ensuring stable charging/discharging.
However, the SEI film is insufficient to play a role of a continuous protective film of an anode, so the life cycle and performance of a battery are deteriorated as the battery repeats charging/discharging. In particular, the SEI film of a lithium secondary battery is thermally unstable. Thus, if a battery is operated or left alone under a high temperature circumstance, the battery may be easily collapsed due to electrochemical energy and thermal energy, which are increased as time goes. For this reason, the battery performance is further deteriorated under a high temperature circumstance.
In order to solve the problems discussed above, non-aqueous electrolyte solutions having various additives have been proposed.
Korean Laid-open Patent Publication No. 2003-59729 and Japanese Laid-open Patent Publication Nos. 2003-323915, 2002-134169 and 2003-173816 disclose a non-aqueous electrolyte solution containing a siloxane compound such as 1,3-divinyltetramethyldisiloxane. As disclosed in these documents, if a predetermined siloxane compound is added to a non-aqueous electrolyte solution, life cycle and low temperature characteristics of a battery are improved. However, in case a lithium secondary battery adopting a non-aqueous electrolyte solution to which a predetermined siloxane compound such as 1,3-divinyltetramethyldisiloxane is added is used for a long time, the capacity of the battery may be degraded.